IF the hall of your ancestral home is graced with a barometer, and if you have looked at it in recent days, you may have noticed that it read exceptionally high. An average pressure value has the needle hovering around 12 o'clock; earlier this week the pressure over Ireland was close to 1045 millibars, or hectopascals as we like to call them nowadays, which would leave the pointer steady near "a quarter past".
The average value of atmospheric pressure over Ireland is about 1013 hectopascals. It is generally higher in summer than in winter, with monthly averages varying from a low of 1011 hPa in December and January, to a high of about 1016 in mid summer. It is something of a paradox, however, that extremely high pressure values occur, almost invariably, in wintertime.
High values of atmospheric pressure are associated with anticyclones. At the risk of oversimplification, one could say that these tend to form where the atmosphere is relatively cold. Cold air is dense, and therefore heavy, and the column of air above a particular spot will weigh more than a corresponding column elsewhere if it contains more than its fair share of relatively cool air; the extra weight causes the pressure at the surface to be higher than the average.
For these reasons, summer anticyclones tend to form over the relatively cool ocean areas of the world. In wintertime, on the other hand, high pressure is more likely to be found over the cold continents especially over the vast land areas of eastern Russia where the Siberian winter anticyclone is a dominant feature of the global weather pattern.
Not surprisingly, therefore, it was there in Siberia that the highest pressure ever measured was recorded - 1084 hPa on December 31st, 1968, at a place called Agata when the temperature was a chilling -46 C. Five other weather stations in the vicinity recorded similarly high pressure values, thus discounting any possibility of error.
Coming closer to home, and looking at measurements in Britain and Ireland, we see that the range of pressure values is more modest. The highest ever recorded in these islands was 1054 hPa in Aberdeen, 95 years ago today on January 31st, 1902.
Here in Ireland, the record is 1052 hPa, seven hPa higher than of late, and noted at Valentia Observatory, Co Kerry, on January 20th, 1905. And for the record, so to speak the lowest pressure ever measured on this island was that which occurred on December 8th, 1886, in Belfast, when the barometer dropped to 927 hPa.